In general, when tightening a number of tightening members onto one tightening object, tightening force to be imparted to the tightening members must be equalized in order to assure the entire tightening strength. However, if the same worker uses the same tool for tightening the tightening members, it is difficult to equalize the tightening force to be imparted to the tightening members.
Furthermore, the tightening member secured to the tightening object is sometimes slackened due to some subsequent events (deterioration, fatigue, desiccation, etc.) happened on the tightening object.
Under the circumstances, a need has been felt for development of a tightening device capable of uniformly tightening many tightening members and preventing the tightening object tightened with tightening members from loosening over a longer span of time.
There has been known in the prior art a tightening device satisfying the aforesaid requirements, as disclosed in PCT Patent Application Publication No. WO00/01/01509.
This conventional tightening device comprises an engaging part engaged with the tightening member, which is integrally disposed on one end portion of an energizing member for energizing the tightening member to be secured onto the tightening object in the tightening direction, and a switching member connected to the energizing member for switching over to its constraining or operating state of the energizing force imparted to the tightening member. The switching member is fitted to the outer periphery of the energizing member formed of a spiral spring, so that the energizing force is prevented from being imparted to the tightening member by constraining elastic expansion of the energizing member in its fitted state (constraining function),  and the energizing force is imparted to the tightening member by permitting elastic expansion of the energizing member in the state of releasing (breaking or the like) the fitted state (operating state).
When using the conventional tightening device, the switching member is engaged with the energizing member, which retains the energizing force by compression, and then, upon engaging the engaging part of the energizing member with the tightening member, the engagement of the switching member with the energizing member is released to impart the energizing force to the tightening member.
The conventional tightening device has an advantage in that it is simple in structure and easy to handle.
However, the conventional tightening device has a disadvantageous possibility of releasing the engagement of the switching member with the energizing member where the energizing force is strong, because the switching member is merely engaged with the outer periphery of the energizing member, thus unsteadily hindering the expansion of the energizing member. Besides, the switching member in its released state stays behind around the outer periphery of the energizing member, thus leading to inhibition of the energizing function of the energizing member and causing a corrosion around the energizing member.